1. Field of the Invention
The present invention relates to microporous ceramic materials and methods for making the same.
2. Description of the Related Art
Various microporous ceramic materials are used as a gas separating material for separating a desired component (gas molecules), from a mixture of gases; a filter for purifying a liquid or a gas, or a support, onto which a catalyst or other functional compounds are fixed. In particular, ceramic materials having a large number of micropores that are so small as to have an average pore size in the order of nm, are required for use in gas separation and high performance filtration such as molecular sieves. Conventionally, siliceous materials were used as microporous ceramic materials that can meet such a requirement. For example, Japanese Laid-Open Patent Publication No. 10-85568 discloses a microporous silica membrane having an average pore size of about 1 nm, a high gas permeability rate and gas selective permeability.
When using a microporous ceramic material as a gas separating material (e.g., gas separating membrane) or a filtration device, a high level of heat resistance (thermal strength), is required depending on the state where it is used. For example, a ceramic material for hydrogen separation which has high hydrogen separation properties at high temperatures (e.g., 400° C. or higher, preferably 600° C. or higher), that are as high as at room temperature is desired as a ceramic material for hydrogen separation (hydrogen separating membrane), that can separate selectively hydrogen from a mixed gas containing hydrogen. However, the microporous ceramic material made of silica is not suitable for use at such high temperatures. A microporous membrane made of silica is disadvantageous in that pores tend to disappear at high temperatures of 600° C. or higher and that this microporous membrane tends to be degraded by moisture when it is used in steam reforming.
Microporous nonoxide ceramics containing Si—C bonds, Si—N bonds and the like have been under development as a preferable material for use under high temperature conditions. For example, Japanese Laid-Open Patent Publication Nos. 8-104580 (U.S. Pat. No. 5,643,987), 8-165177 (U.S. Pat. No. 5,563,212), 10-500655 (U.S. Pat. No. 5,696,217) and 2000-507198 (U.S. Pat. No. 5,872,070) disclose nonoxide microporous ceramic materials that can be used at a relatively high temperature (400 to 500° C.). There is a demand for microporous ceramic materials that can perform gas separation or high performance filtration efficiently at temperatures (e.g., 600 to 700° C.) higher than that for the conventional microporous ceramic materials disclosed in the above-mentioned publications.